1. Field of the Invention
The present invention relates to a method of generating a two-level pattern for lithographic processing by multiple beamlets. The invention further relates to a computer readable medium for performing, when executed by a processor, such a method. The invention further relates to a pattern generator arranged for performing such a method. The invention further relates to a charged particle multi-beamlet system for exposing a target using a plurality of beamlets, in which the system comprises such a pattern generator. Finally, the invention relates to a lithographic system comprising such a pattern generator.
2. Description of the Related Art
Systems using a black and white writing strategy, i.e. an “on” and “off” writing strategy, are widely known in the art. They may use, for example, laser beams or charged particle beams, and may feature the use of direct writing in maskless systems. By modulating the beam (or beams in multi-beam systems), individual grid cells in a rasterized virtual grid may be exposed or not exposed to write the desired pattern on to the target. Such beams are characterized by a so-called beam effect in the target surface, which is often described by a point spread function. The point spread function generally has a Gaussian distribution, which describes the extent of the surface area affected by a beam. The beam size is generally defined as the size of the distribution in which 50% of the beam energy is present.
Generally, the spot area of the beam at the surface area is much larger than the typical size of the grid cells. A full exposure of a certain grid cell thus also causes an exposure with less intensity in the grid cells adjacent to the exposed cell. So, in case of a charged particle beam, the number of charged particles deposited within an individual grid cell, also referred to as dose, constitutes of the sum of the dose received directly from exposure of the grid cell itself and indirectly from exposure of adjacent cells. By selecting a suitable cut-off level for development of the resist layer being exposed, desired feature dimensions can be obtained.
A particular kind of charged particle beam based lithographic system is known from U.S. Pat. No. 6,897,458, assigned to the present owner of the invention, and involves a massive plurality of charged particle beamlets generated in a charged particle beam column for exposing a target. The charged particle beamlets are scanned over the target while being modulated. Additionally, the target may be capable of moving relative to the beams, for example in a direction transverse to the scanning direction of the beams. The modulation of the beamlets is performed on the basis of pattern data provided to the lithographic system. In the particular system described, the modulation is performed by blanking or blocking beamlets to effectively switch the beamlets on and off.
Exposing a target using this type of lithography system is achieved by the combination of relative movement of the target and modulation (e.g. timed “on” and “off” switching or blanking) of each charged particle beamlet. A known method to expose a substrate with beamlets is a raster scan method. In order to control the beamlets in such a scanning method, the pattern data is rasterized. The target is positioned on a motor driven stage that is moved in a continuous motion. As the stage is moved, the beam is scanned in a direction substantially perpendicular to the stage motion. By supplying the rasterized pattern data to the system, timed so that the beamlets are modulated in synchronism with the beamlet deflection and stage motion, the pattern represented by the pattern data can be transposed as an exposure pattern onto the surface of the target. The rasterized pattern data corresponds to an exposure pattern on a virtual raster cell grid on the surface of the target.
Existing charged particle beam technology is suitable for lithography systems for relatively course patterning of images, for example to achieve critical dimensions (CDs) of 90 nm and higher. However, a growing need exists for improved performance. It is desired to achieve considerably smaller critical dimensions, for example 22 nm, while maintaining sufficient wafer throughput, e.g. from 10 to 60 wafers per hour or higher.
In a conventionally rasterized pattern as discussed above feature placement is limited to the grid lines of the raster cell grid. However, due to for example correction rules needed to correct for several resolution-disturbing phenomena like the proximity effect, edges of a feature often do not necessarily fall on a grid line. For this reason, a tendency exists to choose the raster cell grid as small as possible.
However, in particular in charged particle beam systems using a plurality of beamlets, a grid size as large as possible is desired in view of data processing constraints. International application WO2007/105939, assigned to the present owner of the invention, addresses the issue of choosing a suitable grid size by introducing the use of “ragged” edges to enable placement of feature edges between grid lines.
A further difficulty of patterning with a plurality of beamlets is dose variation between different beamlets. In a charged particle system, the current per beamlet generally varies. In multibeam systems, different parts of a substrate to be patterned are exposed by different beamlets. As a result of beamlet dose variation, patterning errors may occur. A writing strategy as presented in WO2007/105939 is unable to resolve this issue.